1. Field of the Invention
This invention relates to a laser optical system. and more specifically to a laser optical system which is suitable for use in an optical recording apparatus adapted to record information such as text or the like on a recording medium using lasers such as a phase synchronizing semiconductor laser and a broad contact semiconductor laser which oscillate in transverse multimodes.
2. Description of the Related Art
A phase synchronizing semiconductor laser has been constructed as a multistripe laser diode array, as illustrated in FIG. 2, in which stripe-shaped waveguides are used. Namely, from several to several tens of laser oscillating regions 12 are arranged parallel in a common single plane, by dividing an active layer which is formed in parallel with a plane of a pn junction 10 into a plurality of stripes, thereby forming a multiple hetero-junction structure. In the above-described structure, each laser oscillating region operates at a fundamental transverse mode. Each adjacent laser oscillating region is coupled together at the energy level. The phases of laser beams emitted from the respective laser oscillating regions are synchronized, so that they oscillate in transverse multimodes to provide a high output. Two suitable semiconductor lasers are the SDL-2410 and SDL-2420 series (manufactured by Spectra Diode Laboratories, Inc., Trademark), etc.
There is, however, a phase difference between laser beams which are emitted from the respective laser oscillating regions of such a semiconductor laser. When the phase difference between laser beams emitted from each laser oscillating region is 180.degree., it is known that two lobes are formed in a direction parallel to the plane of the pn junction in a far field pattern as illustrated in FIG. 3. Even when laser beams capable of forming two such lobes are used to record on a recording medium, the laser beams do not converge toward one spot. It is not therefore possible to create an optical system having high resolution. When it is desired to record information on a microfilm by using laser beams to create dots, it is necessary to obtain resolution in the order of 3360 dots/7.2 mm. Hence, to record dots at very high accuracy, poses a problem.
The conventional solution to such a problem has been to propose that one lobe be cut off by making use of each optical system shown in FIGS. 4(A) and 4(B) [Appl. Phys. Lett. 41 (12), 15 Dec. 1982]. A phase synchronizing semiconductor laser 14, a spherical lens 15, an aperture 18 extending in a direction perpendicular to the plane of the pn junction, a cylindrical lens 20 arranged to converge laser beams emitted from the laser, which are parallel with the plane of the pn junction and a pherical lens 21 are arranged in order in such an optical system. Laser beams emitted from the phase synchronizing semiconductor laser 14, which are in a direction perpendicular to the plane of the pn junction, are rendered parallel as illustrated in FIG. 4(A) and then pass through the aperture 18 and the cylindrical lens 20, so that they are focused on a recording media 24 through the spherical lens 21. These laser beams are parallel to the plane of the pn junction and are focused on an imaginary spot of the aperture 18 by the spherical lens 15 as shown in FIG. 4(B), so that well-defined far-field patterns are formed on the focal point. The aperture 18 passes only those laser beams which form one of the lobes while cutting off those beams which form the other lobe. The laser beams which pass through the aperture 18 are rendered parallel by the cylindrical lens 20 so that they are focused on the recording medium 24 through the spherical lens 21. Well-defined spots with diameters of several micrometers can therefore be obtained.
However, since one of the laser beams capable of forming the lobes has been cut off, the above-described conventional optical system is accompanied by a problem in which the light intensity of the laser beams which are emitted in the direction parallel to the plane of the pn junction is halved. The resulting poor efficiency makes such a laser beam inadequate for applications which require high energy during recording, e.g., heat-mode recording materials such as a laser direct recording film.
On the other hand, Japanese Patent Application No. 98320/1987 discloses that two lobes are separated subsequent to the formation of laser beams into a bundle of parallel rays to combine the two lobes into one by making use of a reflecting mirror, a 1/2 wavelength plate and a polarizing beam splitter. The use of the optical system which is disclosed in the application referred to above makes it difficult to obtain complete parallel rays in practice. Where each optical path of the respective lobes is not equal, the optical position of a beam waist to be formed upon convergence through the last lens is deviated from the optical axis, thereby making it difficult to converge the laser beams at a high energy density.
In a broad contact semiconductor laser having plural memories width optical waveguides, where each width of the stripes have at least a predetermined value which corresponds to 10 micrometers or greater, laser beams having high outputs of several hundreds mW to 1 W can be obtained. Less restriction on transverse directions is, however, made owing to the broad width of each stripe to multimode transverse modes. Such a broad contact semiconductor laser is therefore accompanied by a problem in which laser beams cannot be focused on a spot in the order of several microns as in the abovedescribed phase synchronizing semiconductor laser. Incidentally, there are two types of broad contact semiconductor lasers, one making use of refractive index-guiding and the other gain-guiding.